Reactive oxygen species (ROS1) are generated constantly in vivo from ground state triplet oxygen. This occurs by a variety of endogenous processes, including normal mitochondrial aerobic respiration, phagocytosis of bacteria- or virus-containing cells, and peroxisomal-mediated degradation of fatty acids. The ROS are normally detoxified by antioxidant defense systems, such as superoxide dismutase, catalase, and GSH dependent peroxidases. Many other endogenous processes protect against ROS-mediated damage including the sequestration of hydrogen peroxide generating enzymes and the chelation of free transition metal ions by transferrin, ferritin, and ceruloplasmin.
Oxidative stress occurs as a result of increased ROS production during inflammation, radiation, or the metabolism of hormones, drugs, and environmental toxins. This overwhelms endogenous protective mechanisms and increases ROS-mediated lipid peroxidation, which results in damage to cellular macromolecules. Lipid hydroperoxide-mediated damage to cellular macromolecules can also arise from oxidative stress induced by cyclooxygenases (COXs) and lipoxygenases (LOXs). ROS-mediated peroxidation of free linoleic acid (LA) and arachidonic acid (AA) results in the formation of complex mixtures of hydroperoxyoctadecadienoic acids (HPODEs) and hydroperoxyeicosatetraenoic acids (HPETEs) that are reduced to racemic hydroxyoctadecadienoic acids (HODEs) and hydroxyeicosatetraenoic acids (HETEs), respectively. Lipid hydroperoxides are also formed by LOXs and COXs.
These enzymatic pathways result in a much simpler profile of HPODEs and HPETEs. LA is converted primarily to 13(S)-HPODE by human 15-LOX-1 and 15-LOX-2, and COX-1 and COX-2 mainly produce 9 (R)-HPODE and 13(S)-HPODE from LA. The HPODEs are reduced to the corresponding 9 (R)- and 13(S)-HODEs by intracellular peroxide reducing enzymes. With AA as substrate, COX-1 and COX-2 both produce 15(S)-HPETE. The 15(S)-HPETE is reduced to 15(S)-HETE through the peroxidase activity of COXs or by GSH-dependent peroxidases.
Lipid hydroperoxides undergo homolytic decomposition to the bifunctional electrophiles 4-oxo-2(E)-nonenal (ONE) and 4-hydroxy-2(E)-nonenal (HNE). ONE and HNE both contain an R″-unsaturated aldehyde. However, ONE is much more efficient than HNE at modifying DNA through the formation of heptanone-etheno (HFε) adducts. ONE and HNE also form adducts with amino acid residues such as lysine and histidine in proteins. 15(S)-HPETE undergoes vitamin C- and transition metal ion-mediated homolytic decomposition to ONE and HNE in a manner similar to that of 13(S)-HPODE. COX-2-derived 15(S)-HPETE is responsible for ONE-mediated formation of Hε-2′-deoxyguanosine (dGuo) adducts in the DNA of rat intestinal epithelial (RIE) cells that stably express COX-2 (RIES). Furthermore, Hε dGuo and Hε-2′-deoxycytidine (dCyd) adducts were found in intestinal polyps from min mice, a mouse model with increased intestinal COX-2 expression.